Combustion of gases



Sept. 17, 1 6 UNTERSTENHOEFER ETAL 3,401,530

COMBUSTION OF GASES Filed Dec. 7, 1966 INVENTORS LEO UNTERSTENHOEFER CARL HEZEL ATT'YS United States Patent 3,401,680 COMBUSTION OF GASES Leo Unterstenhoefer, Limburgerhof, Pfalz, and Carl Hezel, Ludwigshafen (Rhine), Germany, assignors to Badische Anilin- & Soda-Fabrik Aktiengesellschaft, Ludwigshafen (Rhine), Germany,

Filed Dec. 7, 1966, Ser. No. 599,889

Claims priority, application Germany, Dec. 8, 1965,

6 Claims. (Cl. 126-91) ABSTRACT OF THE DISCLOSURE Apparatus for combustion of fuel gas with air including a burner tube arranged coaxially within an elongated heating tube, an inner tube inserted within the burner tube and capped at its outlet end with a glow plug which is located adjacent the outlet of the burner tube, inlet means for introducing air into the heating tube, inlet means for introducing fuel gas into the burner tube, and an annular constricting means located downstream from said inlet means in each of the annular channels defined by the three coaxially arranged tubes. The apparatus provides adequate damping of vibrations which are caused by the combustion flame.

This invention relates to improvements in the combustion of gases. More specifically it relates to an improved apparatus for burning gases, for example illuminating gas or natural gas, with air in a long tubular combustion chamber. Heating tubes of this kind may be used for indirect heating of liquids, gases or fluidized beds. Heat liberated by burning the fuel gas is transferred by radiation and contact to the wall of the heating tube and given off, mainly by convection, by the outer surface of the tube to the material to be heated. Such tubes are known as convection heating tubes.

Whereas the length of convection heating tubes may be about one hundred times the diameter of the tube or more, radiation heating tubes are usually shorter because the wall temperature of the tube cannot be kept over an unlimited length at such a height that favorable heat radiation can be effected. Moreover very high temperatures cannot be sustained by the tube material for long periods. For the same tube diameter, the possible throughput of fuel gas in the case of convection tubes is usually a multiple of that of radiation tubes because in the former the transfer of heat to the material being heated is better and the loading of the heating surface may therefore be kept very much higher without the tube material being overheated.

Vibrations may be caused by the flame in every combustion. The intensity of these vibrations varies depending on the type and volume of the combustion chamber including the olfgas pipes, i.e., the gas column capable of being excited. They may be intentional and then are utilized, for example in the so-called Schmidt tube, to draw in the fuel and to expel the oflgas. Usually these vibrations are disadvantageous however because they aflect the mixing of the combustion gases and the combustion itself and because the noise associated with the vibrations is very troublesome, for example in oil firing plants.

In combustion in a tube, the contents of the tube are set in vibration by the flame so that at an appropriate exciting frequency they may vibrate in resonance. The longer the heating tube, the lower the frequency of the resonance vibration and the more troublesome the resonance vibration, because it readily extinguishes the flame. If the throughput of gas is increased, with consequent increase in the energy converted in the flame, the vibrations be- 'ice come stronger. A heating tube having an internal diameter of mm. and a length of 10 m. can be operated satisfactorily up to an illuminating-gas throughput of 15 m5 (S.T.P.) per hour. However, the result of an increase in the gas throughput is that the noise increases considerably, combustion becomes irregular and the flame begins, at a gas throughput of about 25 m. (S.T.P.) per hour, to move away from the burner throat and to become unstable. The main reason why the use of convection tubes results in difficulties is that the temperature of the tube Wall is lower than in the case of radiation tubes. The hot wall of the radiation tube, which is usually above 700 C., has a stabilizing effect on the flame. For this reason a heating tube having a lower wall temperature is always noisier than the same type of tube having a wall temperature of more than 600 C.

Apparatus for burning gases in heating tubes are known in which the length of the flame may be varied, for example from outside, or in which special measures are taken to ensure uniform transfer of heat to the jacket tube. An arrangement is known in which the heating tube is passed underneath water as a dip tube and a branch stream is ignited as an ignition stream above the level of the liquid, the main stream of the mixture of gas and air not being brought together with the ignition stream and thereby ignited until they are beneath the water.

It is an object of the invention to provide an improved apparatus for the combustion of fuel gas with air in a heating tube in which the length of the heating tube is a multiple of the diameter of the tube and a horizontal burner tube having ignition means is located in the interior of the heating tube.

Another object of the invention is to provide an apparatus for burning large amounts of gas satisfactorily in long convection heating tubes or in shorter radiation heating tubes.

These objects are achieved in accordance with this invention by increasing the velocity of the fuel gas and/or the air upstream of the outlet from the throat of the burner tube and thereafter decreasing it again.

The velocity of the gas and air or a mixture of the two is increased to at least one third of the speed of sound in order to achieve adequate damping of the vibrations.

Depending on the properties of the fuel gas, either only the velocity of the combustion air or only the velocity of the fuel gas is raised and thereafter lowered again, gas or combustion air being supplied to the combustion zone at unchanged velocity.

It is possible to add a portion of the combustion air, for example 5 to 35% by volume, to the fuel gas prior to its entry into the combustion zone and to mix it therewith.

To increase heat transfer, the velocity of the offgas downstream of the combustion zone may be increased for a short time.

Apparatus for burning fuel gas with air in accordance with this invention consists of a heating tube, a pipe connection on the heating tube for the supply of air, a burner tube introduced coaxially into the heating tube, and a second pipe connected on the heating tube for the supply of fuel gas to the burner tube, wherein there are provided coaxially within the burner tube an inner tube, a glow plug at the outlet of the inner tube, an annular constriction member on the outer surface of the inner tube and/ or an annular constriction member on the outer surface of the burner tube, and orifices and air supply pipes in the wall of the burner tube.

The burner tube and the inner tube flare out into lengths of increased diameter.

In the offgas pipe which serves as an extension of the burner tube, a nozzle like constriction is provided having a free cross-sectional area of about one quarter of that of the heating tube.

A movable thermocouple is provided in the burner chamber for measuring the temperature of the flame.

An embodiment of apparatus in accordance with this invention is shown in the accompanying drawings and will now be described in detail. FIGURE 1 shows, in section, a burner for a heating tube, FIGURE 2 is a detail in section of part of the heating tube and FIGURE 3 is a burner heating tube unit for a fluidized-bed reactor.

The burner for combustion of fuel gas with air consists for example of a burner member 1 having a burner flange 2, an air pipe connection 3 having a flange 4 and a burner base 5 (FIGURE 1).

A heating tube flange 6 carrying a heating tube 7 is secured by bolts to the burner flange 2. A burner tube 8 and an inner tube 9, arranged concentrically, are secured to the burner base 5. The burner tube has a flared portion 10 to which is attached an end piece 11 having a burner throat 12. Analogously to the burner tube with its parts 10 and 11, the inner tube 9 has a flared portion 13 having secure to it an end piece 14 which is closed at the end with a conical member 15. The conical member 15 has a bore 16 in which a glow plug 17 is secured.

The interspace 18 between the heating tube 7 and the burner tube 8 serves for the supply of air. The fuel gas is supplied through the interspace 19. The space 20 in the inner tube 9 serves for the reception of current leads for the supply of current to the glow plug 17. The interspace 18, with the flame at the burner throat 12, may

be observed through a sight glass 21. Constriction members 22 and 23 are provided on the burner tube 8 between the fiared portion 10 and the burner throat 12 and on the inner tube 9 in the vicinity of the flared portion 13. A thermocouple 24 serves to measure the temperature within the calming zone 32.

The lengths of the individual portions of the burner tube 8 and the inner tube 9 are chosen so that, when the burner is introduced into the heating tube 7 and the burner flange 2 is connected to the heating tube flange 6, the burner throat 12 and consequently the point at which burning begins is at that point in the heating tube 7 where heat development should begin.

Fuel gas and air are supplied separately through pipes 25 and 3 into the interspaces 19 and 18 serving as annual channels, are free to mix at the burner throat 12, and burn as soon as they are ignited by the glow plug 17. Supply of the ignition current is effected through the inner tube 9 which is open at the burner base 5. Some of the combustion air from the interspace 18 may be mixed with the fuel gas through openings 26 at the flared portion 10 of the burner tube 8. The thermocouple 24, which is capable of being moved axially and whose lead leaves the unit as 27, serves to measure the temperature of the Stabilization of the combustion is achieved by preventing gas oscillations being propagated from the combustion chamber 28 into the pipe connections 3 and 25 filled with air and gas respectively. Experiments have shown that sympathetc vibration of the gas columns in the supply pipes seiously affects mixing and ignition and may cause the burner to go out. The longer the supply pipes, i.e.,

the farther the nearest control member is removed from the point of ignition, the greater the danger that this may happen. Reliable parallel operation of a number of heating tube units with only one common control has hitherto not been possible because long supply pipes are necessary and because the burners tend to influence each other. These problems are overcome by using the process according to this invention.

The vibrations in the combustion chamber are propa gated at the speed of sound. Their propagation upstream to the supply pipes 3 and 25 may be avoided by ensuring that the fuel gas and air, prior to ignition, have to pass through constrictions 29 and 30 at high velocity. Downstream of these constrictions there is in each case a calming zone 31 or 32 the length of which is at least twice to three times the diameter of the tube. Here the gases are brought back to normal velocity before they mix with each other at the burner throat 12 where they are ignited.

The Zones 31 and 32 between the constrictions 29 and 3t) and the burner throat 12 should be short. If the disstance between the constriction and the burner throat is increased, the effect of the constriction decreases; on the other hand, the constrictions should not be too near to the burner throat 12, Where ignition takes place; otherwise mixing of the gases is too vigorous owing to their high turbulence at the burner throat and in consequence a spontaneous combustion reaction takes place which is not desirable because of the risk of the tube burning through. The speed in the constrictions 29 and 30 should be as high as the speed of sound in the gas concerned in order to achieve complete damping of the vibrations. It has been found however that damping is adequate at a speed of about one third of the speed of sound. The speed of the air and the speed of the fuel gas in the respective constrictions may be different. In the case of short heating tubes, it is suflicient to provide a single constriction in the path of the combustion air in order to ensure satisfactory operation.

The process according to this invention also makes it possible to mix a portion of the combustion air (which can be exactly regulated and which is preferably 5 to 35% so as to remain outside the ignition limits) with the fuel gas as primary air upstream of the point of ignition. This enables the length of the flame in the heating tube to be adjusted in known manner to the most favorable value for each type of heating tube. For this purpose it is suificient to arrange the constriction members 22 and 23 in the burner so that the pressure building up upstream thereof may be utilized to force some of the combustion air through openings from the interspace 18 into the fuel gas. As shown in FIGURE 1, the primary air at the flared portion 10 may be passed through openings, for example through openings 26 distributed uniformly around the circumference, and then mixed with the fuel gas from the interspace 19 so that a mixture of fuel gas and air is present in the calming zone 31. The air may however, as shown in FIGURE 2, be passed from the flared portion 10 through air supply pipes 33 to a point just upstream of the burner throat 12 where it is mixed with the fuel gas. Both methods are possible and result inter alia in the advantage that the burner can be ignited more easily.

FIGURE 3 shows the burner heating tube unit of a fluidized bed reactor 34 which serves to keep a fluidized bed 35 at a constant temperature, for example 400 C. The fuel gas supply pipe connection 25 and the air supply pipe connection 3 of the burner member 1 are situated outside the reactor 34 and the point of ignition is approximately at the height of the sieve plate 36. The otfgas is Withdrawn through an otfgas pipe 37 through a recuperator (not shown) in which the heat of the offgas is utilized.

If a constriction, for example in the form of a standard jet having a free passage area of for example one quarter of the area of the offgas tube, is provided in the oflgas pipe 37, the combustion vibrations are additionally damped. For example a heating tube having an internal diameter of mm. has a noise level of 114 decibels in the offgas tube at a gas throughput of 60 m. (S.T.P.) per hour; after installing a standard baflle having an internal diameter of 50 mm., the intensity of the sound is only 101 decibels.

If a plurality of single burners as shown in FIGURE 3, for example sixteen heating tubes having an internal diameter of 100 mm., be installed in a reactor and connected in parallel so that the temperature of the reaction can be kept constant by regulating the valve for the total amount of gas, it is to be expected that the individual burners either immediately upon ignition or during burning will disturb and extinguish each other. If however the combustion gases in each individual burner are passed through constrictions as proposed according to this invention, the sixteen individual burners, each having a tube length of 22 meters, may be ignited singly or simultaneously and each may be operated with a throughput of up to 100 m. (S.T.P.) per hour of illuminating gas and checked individually. Ignition may be eifected with a 12-volt glow plug 17; the thermocouple 24 (FIGURE 1) which projects into the flame measures the temperature of the flame and cuts oil the gas supply to the burner if the flame is extinguished.

We claim:

1. Apparatus for the combustion of fuel gas with air comprising a heating tube, a pipe connection on the heating tube for the supply of air, a burner tube inserted coaxially in the heating tube, a second pipe connection on the heating tube for the supply of the fuel gas to the burner tube, an inner tube arranged coaxially within the burner tube, a glow plug at the outlet of the inner tube, an annular constriction member on the outer surface of the burner tube, and an annular constriction member on the outer surface of the inner tube.

2. Apparatus as claimed in claim 1 provided with openings in the burner tube to permit admission of air from the heating tube into the burner tube.

3. Apparatus as claimed in claim 1 provided with openings in the burner tube to permit admission of air from the heating tube into the burner tube and with pipes in the burner tube connected to the said openings to conduct air from the heating tube into the burner tube at points in the vicinity of the glow plug.

4. Apparatus for the combustion of fuel gas with air comprising an elongated heating tube, inlet means to introduce air into said heating tube, a burner tube arranged coaxially within said heating tube to provide an annular interspace for the flow of the introduced air and having a coaxial burner throat extension attached at its terminal end by an outwardly flaring joint, inlet means to introduce fuel gas into said burner tube, an inner tube arranged coaxially within said burner tube to provide an annular interspace for the flow of the introduced fuel gas, a glow plug capping the outlet end of the inner tube and mounted within said burner throat extension adjacent the outlet thereof, and an annular constricting means located downstream of said inlet means in each of the annular interspaces defined by the three coaxially arranged tubes.

5. Apparatus as claimed in claim 4 wherein said outwardly flaring joint of said burner tube is provided with openings to permit admission of air from the annular interspace of said heating tube into the annular interspace of said burner tube.

6. Apparatus as claimed in claim 5 wherein said openings are connected to pipes extending longitudinally within the annular interspace of said burner throat extension to conduct air from the heating tube into the burner tube at points in the vicinity of the glow plug.

References Cited UNITED STATES PATENTS 5/1957 Monkowski 126-91 9/1959 Blackman 126-91 

